For Unity, their first-party and third-party tools work pretty well. Even in combination, if you find the right ones.
Unity's new Tilemap feature and the third party Ferr2D Terrain's bezier terrain system are pretty good for sidescrolling and work well together. It takes care of a lot of the graphical and game-physics-shapes aspects that you'd expect from level editing.

But as general-purpose as these are, I would still need to code a LOT of extra stuff to make it suitable for level design for a specific 2D platformer, like jump rulers and spacing rulers, and interactible stuff. A lot of the stuff I have to add on top of it rely on parameters that I can't expect the tool to know ahead of time, like how gameplay rules work or how jumping or wall-interaction physics is implemented.

In general, I think level editors either need to assume and restrict a lot (like RPG Maker type stuff) so it can be convenient and ready to use. Otherwise, they are so broad and generic that you can add anything to it but then you'd have to do the actual coding for the additional stuff.

I know that's a lot of obvious stuff. I mention these for the newbie readers who might pass by. Hello, friends!

thanks for your answers guys!